1. Fields of the Invention
This invention generally relates to electron emission devices and more particularly to a multiple electrode field electron emission device which controls field emitted electrons originating from a cold cathode. More specifically, this invention relates to a multiple electrode field electron emission device having a linearly related input signal voltage and anode current, which is useful and/or advantageous in applications, such as, power amplifiers, linear amplifiers, and switching circuits.
2. Related Technology
An example of a multiple electrode field electron emission device is reported by Junji Ito in Journal of Applied Physics, Vol. 59, No. 2, on pages 164 to 169 (1990). A generalized illustration of such a multiple electrode field electron emission device is shown in FIG. 33, and is referred to as a flat triode emission device. A wedge shaped emitter electrode (cathode) 102 and a gate electrode 103, which is a column, and an anode 104 are sequentially fabricated on one surface of a quartz substrate base 101. The three electrodes are formed by using a photo-etching process to deposit and shape a thin tungsten film about one micron thick. The emitter electrode 102 has a series of about 170 electrode elements or tips which have a pitch of 10 microns and form a linear array. The separation distance between gate electrode 103 and emitter electrode 102 is 15 microns and between the gate electrode 103 and anode 104 is 10 microns.
When the electrical properties of this triode structure are measured in a vacuum of 5.times.10.sup.-6 pa, the emitter exhibits a Fowler-Nordheim (F-N) tunnel current type emission current. With the gate and anode voltages set at 220 V and 318 V, respectively, an anode current of about 1.2 microamps is obtained. This amounts to about 7 nA of anode current for each emitter electrode element and the mutual conductance for these elements is about 0.1 .mu.S.
However, such a triode device structure has a number of potential problems. While electrons emitted from electrode 102 proceed toward anode 104 some flow to positively biased gate electrode 103 because of its intermediate position between the two other electrodes. Because the gate current is equal to or higher than the anode current, the gate input resistance is very small. That is, the electron yield (anode current divided by total emission current) flowing to anode 104 decreases, causing a reduction in electrical properties because characteristics such as power efficiency and mutual conductance are reduced. Therefore, this technology provides about a 60% yield. When controlling anode current with a low gate input resistance triode device, it is necessary to provide a circuit or isolation element to accommodate large current and power in signals input to gate electrode 103. This limitation makes it difficult to use current triode devices as current amplifiers and power switches.
In addition, the triode emission current is in the form of Fowler-Nordheim (F-N) tunnel current, which increases or decreases exponentially relative to changes in the gate voltage. As a result, the anode output current relative to the gate input signal changes exponentially. Triode devices possessing this type of non-linear input and output relationship, are not capable of being used for applications such as linear amplifiers.
Furthermore, in order to increase performance by enlarging the mutual conductance of the triode device, it is necessary to modify the gate electrode 103 structure and enlarge the emission surface area of emitter electrode 102. However, enlarging the emission surface area also increases the number of electrons flowing to gate electrode 103. Therefore, a power amplifier with high performance can not be obtained using current technology.
Cathode 102 and gate electrode 103 are typically fabricated during the same photo-etching process. Electrode separation is determined by the resolution of the photoresist process or exposure, and is practically limited to 0.8 microns. Furthermore, as process geometries become smaller, variations increase. The magnitude and uniformity of the threshold voltage for electron emission in the field electron emission device largely depends on the cathode 102 and gate electrode 103 separation. As a result, reducing threshold voltage in current triode devices is difficult and even when successful provides poor uniformity.
The threshold voltage of the field electron emission device also depends chiefly on the radius of curvature for the tip of the cathode 102 elements. That is, the smaller the radius of curvature for an electrode tip, the lower the threshold voltage. To obtain a practical threshold voltage, it is desirable to have a tip radius of curvature of 1000 angstroms or less. However, fabrication of a practical tip radius of curvature is difficult with current processing technology, which is generally limited to 2000 angstroms because of photoresist seepage.
Therefore, the present invention was developed to overcome these problems found in the art. Purposes and objectives of the invention include offering a high performance multiple electrode field electron emission device with a large gate input resistance, a linear input and output relationship, a large mutual conductance, and offering a manufacturing process for such a multiple electrode field electron emission device.